The present invention is concerned with a process for the permethylation of so-called "non-a-tocopherols" to .alpha.-tocopherol using methanol or an equivalent gaseous mixture under near-critical or super-critical conditions and using a particular catalyst.
As is known, the naturally occurring non-.alpha.-tocopherols, .beta.-, .gamma.- and .delta.-tocopherol, differ from .alpha.-tocopherol (which has the highest vitamin E activity and which is accordingly the biologically most valuable tocopherol), by the absence of one or two methyl groups in the 5- and/or 7-position of the chromane part of the molecule. Accordingly, there is a need to convert such non-.alpha.-tocopherols into .alpha.-tocopherol chemically, the main problem lying in the efficient, complete mono- or, respectively, dimethylation of the benzene ring of the substituted chromanyl group.
Synthetic processes for the manufacture of nature-identical .alpha.-tocopherol have hitherto been found to be uneconomical. Natural, especially plant, sources of tocopherols usually contain predominantly non-.alpha.-tocopherols in addition to a relatively low content of .alpha.-tocopherol. For this reason, the isolation of .alpha.-tocopherol from such natural materials (raw materials) is also uneconomical. Therefore, the object of the present invention is to provide a process for the conversion of non-.alpha.-tocopherols, which may be present in appropriate raw materials or obtained from these, into .alpha.-tocopherol. The process of the present invention is in many different respects more economical than previous processes for this purpose.
Some processes for the conversion of non-.alpha.-tocopherols into .alpha.-tocopherol are already known from the state of the art. For example, European Patent Publication (EP) 176 690 (Henkel Corporation) discloses a process for the methylation of non-.alpha.-tocopherols using a methylating agent in the gas/liquid phase and in the presence of a metal oxide catalyst. This is a direct, one-stage methylation of the chromane ring, which is to be seen as a complete tocopherol methylation, i.e., a permethylation.
The catalyst used for this purpose is "functionally" defined in EP 176 690 in the sense that any catalyst which is capable of inducing an alkylating reaction can be used; typically it can be a metal oxide or a mixture of several metal oxides in which the metal atom(s) is/are selected from Groups IIA, IIB, IIIA, IVA, IB, VB, VIB, VIIB and VIII of the Periodic Table. The oxides of Be, Mg, Ca, Ti, Zr, V, Mo, Cr, Mn, Tc, Fe, Co, Ni, Zn, Cd, In, Sn, Si, Al, La, Ce, Pr and Nd are indicated to be preferred metal oxides. Such catalysts can be used as such ("neat") or on an inert carrier material and can be produced in any suitable manner, even in situ. In the case of the in situ production method, for example, a metal salt is introduced into the reactor and subsequently reacted or decomposed to the corresponding metal oxide. The methylation can be effected after removal of byproducts and any unreacted reactants.
According to an example of the "external" production of such a catalyst, dry tin hydroxide is added to a solution of ammonium vanadate in aqueous oxalic acid and treated with a solution, likewise added in activated form, of partially polymerized silicon hydroxide to give a precipitate. This is dried, calcinated and shaped by pressing. In a further example of EP 176 690, solid titanium dioxide is added to an aqueous mixture of ammonium vanadate and oxalic acid and the new mixture is heated and dried, then calcinated and shaped by pressing. In both cases there are obtained oxide mixtures which however cannot be designated crystallographically as mixed oxides; rather they are vanadium oxide on a tin oxide/silicon dioxide or, respectively, titanium dioxide carrier. In EP 176 690 there is no teaching of hydrotalcites or hydrotalcite-like metal hydroxycarbonates as possible metal-containing materials from which the metal oxide or mixed oxide catalysts could be produced, not to mention catalysts containing copper oxide (copper belongs to Group Ib of the Periodic Table).
Further, in EP 176 690 a temperature range of about 390.degree. C. to about 470.degree. C. is mentioned as the especially favourable temperature range and the ambient (i.e., atmospheric) pressure is mentioned as the most preferred pressure. Moreover, the use of excess methylating agent or an inert carrier gas, e.g., nitrogen, but not of an additional solvent, is foreseen.